Read the following research paper excerpt and answer the question. According to Mario Pei, slang is common to English as well as to foreign languages (80). Many words that were once considered slang are now words of full, legitimate standing in our language. Some of these words are strenuous, spurious, clumsy, and bogus. Even Shakespeare introduced into current usage such slang terms of his day as hubbub, fireworks, fretful, (to) bump, and (to) dwindle. Pei estimates that about 90 per cent of all slang words eventually become permanent (Johns 12).

What is the author's style?

He is using slang.
He is using an informal style.
He is using a formal style.
He is not serious in tone.